There are three new iPhones in 2018, in three different sizes and prices. The iPhone XR is the more affordable and has the best battery-life of the new iPhones. The iPhone XS is the smallest, and the sleekest. But the iPhone XS Max is the showboat.
If you're looking for the biggest iPhone of 2018 with the biggest screen, look no further: the iPhone XS Max is the larger-screened premium phone many will crave, but only a few can actually afford, or need. Essentially, compared to the iPhone XS you get a larger screen size, more pixels and a battery that lasts longer for $100 more.
So if you're OK with the $1,099 and up price, go for it. It's a beautiful, if unnecessary, luxury.
However, if you want a larger-screen iPhone, but don't want to pay quite as much, you might want to check out the iPhone XR. It's a better bet for value and battery life.
The thing is, though, the 5.8-inch iPhone X (and XS) already pulled off a great feat in fitting more screen in a smaller size. Going back to a bigger phone after that feels like defeating the purpose. But it's a personal choice. I've always liked the 10.5-inch iPad Pro versus the 12.9-inch. I like the 13-inch MacBook over the 15. So, too, the XS and Max. Others feel differently.
(Note: Updated with final battery results and comparisons to the iPhone XR.)
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What does that screen size give you? The display on the XS Max has tons of pixels: 2,688x1,242, in fact. Movies and games, in particular, look great on it. It's a damn nice display... the OLED looks better this time around, and I've enjoyed just looking at it. It's my favorite Apple device display. But when held side by side with the XS, it's not always easy to see the difference. In fact, I found myself mistaking one for the other when I saw both on a table.
The smaller XS' screen resolution is 2,436x1,125 pixels. That's not a huge difference. The Max's extra size can be helpful when editing photos or watching videos, but it's still a long way from the canvas size of any iPad.
Compare that against the iPhone XR and its slightly smaller, lower-resolution (but still totally fine and crisp) display, and the iPhone XS Max is even harder to justify.
One small thing: the notch at the top of the XS Max is the same as those on the X and XS, and it feels like it vanishes more on the larger-bodied phone.
You can see a few more things, sometimes. Much like the Plus iPhones, a few apps have a dual-pane split view -- Notes and Apple's Mail are a few key ones. It all depends on the app, though. Apps need to update to take advantage of this, or add support for the Max's extra pixels. Similarly, some websites enter a more iPad or laptop-like full-web mode in landscape mode on Safari, but I only had luck with a few so far, The New York Times being one of them. Your mileage may vary here. Next to an iPhone XR, you're not really getting much more space (and the XR also works with that split view in a few apps). But the Max display does it better.